1. Field of the Invention
This invention relates to an emission monitoring and control system for material processing apparatus and, more particularly, it concerns a method and apparatus for adjusting the draft of a burner fired material treatment apparatus, such as an asphalt plant, in a manner to optimize burner efficiency and maintain air pollutants within the established limits.
2. Description of the Related Art
In a typical asphalt plant, a process material such as crushed stone aggregate is heated during continuous movement of the material through a rotary drum either prior to or during mixture with various tars and other asphaltic materials. Heat is supplied by an oil or gas burner located at one end of the drum and oriented to bring the products of combustion emanating from the burner into direct contact with the process material contained in the drum. The products of combustion, water vapor and other air suspended materials are withdrawn from the end of the rotary drum opposite from the burner by draft fan in an exhaust system which includes gas cleaning devices such as a cyclone separator for removing solid particulate materials, fabric filters and the like.
The burners used in such plants are usually fueled by oil, natural gas or liquefied gas and in some cases a combination of oil and natural gas or of natural gas and liquefied gas. They have self contained blowers that supply between 40% and 125% of stoichiometric air required for complete combustion. In the case where less than 125% of the air is supplied by the burner blower or blowers, the remaining air must be induced into the combustion zone by the negative pressure developed by the exhaust fan, the quantity etc. The quantity of air drawn in by draft, however, is the product of many unpredictable variables, such as leakage openings between the drum and the draft fan, of which there are many, filter blockages and the like. Also in the operation of such plants, burner firing rate is modulated primarily, if not exclusively, by the processing demand of the plant. If the plant is operated to dry process material, for example, the burner firing rate will vary with the water content of the process material as well as with the rate of processing material feed through the drum. Burner firing rate modulation is effected by varying the fuel supplied to the burner. In cases where less than stoichiometric air is supplied by burner blowers, the air supply is controlled to a limited extent by the burner air damper while the remaining air plus any desired excess air has had limited control usually by maintaining a constant negative pressure within the drum. While total air burners attempt to modulate the required air, to date this had been difficult to achieve without the benefit of oxygen analyzers.
It will be appreciated, therefore, that the nature of operation and the general arrangement of components in burner fired material processing apparatus represented by asphalt plants are not suited for maintaining precise control of air/fuel ratios in the combustion process associated with such apparatus. The proportions of air and fuel in that combustion process, moreover, determine the quantity of combustible air pollutants, such as carbon monoxide, discharged as stack gases to the atmosphere. As a results, asphalt and similar material processing plants have become the subject of numerous regulations directed to avoiding pollution of the atmosphere. Thus, there is a need for improvements in such apparatus by which the air pollutants exhausted to the atmosphere are kept within acceptable limits without sacrifice in material processing capacity or in the economic advantages of existing plants.